Mei and Yomi are adopted cousins and Kagura is considered Yomi's adopted younger sister. In case anyone is curious about the relations between the characters. As to the deeper interpersonal relationships, which are quite intricate, you would have to read the manga or watch the anime.
I wasn't pleased with the seemingly deep departure of what actually happened in this scene in the anime at first nor with how the feelings went from "I'm going to KILL you" to "I won't let anyone else have you." It seemed a poor transition into an obvious porn plot. However the twist at the end redeemed it well for me, I liked the nice tie in and foreshadowing.
Art 5. Plot 6, Translation 6, True to character 4. All out of 10